1. Field of the Invention
The present invention relates to a method of manufacturing a flash memory device. More specifically, the present invention relates to a method of manufacturing a flash memory device, which prevents a smiling effect at a tunnel oxide film arising from a cell region and a gap-filling defect at a field isolation film when the process of self-aligned shallow trench isolation (SA-STI) is adopted therein.
2. Discussion of Related Art
In general, a field isolation film is formed by means of a shallow trench isolation (STI) process or a self-aligned shallow trench isolation (SA-STI) process.
In the STI process, an oxide film and a nitride film are sequentially formed on a semiconductor substrate and an etching process is then performed to form a trench in a field isolation region of the substrate. In the SA-ATI process, an oxide film, a polycrystalline silicon film, and a nitride film are sequentially formed on a semiconductor substrate and an etching process is then performed to form a trench.
In manufacturing a flash memory device using the STI process, a nitride film is formed on a gate oxide film in a periphery region. Accordingly, there is a problem in that an etching damage is occurs at the gate oxide film during removal of the nitride film.
In addition, in manufacturing a flash memory device using the SA-STI process, a polycrystalline silicon film is formed between an oxide film and a nitride film to protect the gate oxide film from etching damage. However, an aspect ratio of a trench is increased due to the polycrystalline film and the aspect ratio of the trench in a cell region having narrow pattern width is increased. Therefore, there are problems in that a filling characteristic of insulating material begins deteriorating and a void is formed in the field isolation film. Also, there is a problem in that a smiling effect because both ends of a tunnel oxide film become thick during subsequent thermal process since the SA-STI process is performed after forming the tunnel oxide film.